Polyphase synchronous motors without a starting cage are linked by the principle of operation to the frequency of the polyphase power supply system that feeds them. Motors such as these therefore cannot be started, that is to say accelerated, directly from the power supply system. In fact, a device which produces a variable-frequency voltage is required between the polyphase power supply system and the synchronous machine. A frequency converter is normally used for this purpose. The frequency converter consists of a rectifier, an intermediate circuit (capacitor) and an inverter.